Crist was crushed in 2010. Some polls show a shallow willingness among Floridians to resurrect his political career — in May, a Florida Opinion Research poll showed him beating Republican Gov. Rick Scott by double digits. But what happens when his rivals in a Democratic primary run the tape of him trashing Obama?

There is a lesson here for political opportunists. Party switches made sense for the conservative Democrats who went to the GOP in the 1990s. They probably made sense for Northeastern Republicans with liberal principles, like Jim Jeffords and Lincoln Chafee, who both left the GOP last decade.

But for Crist, it’s different. He doesn’t have any principles. He may have gotten the Democratic convention speaking slot he wanted, but he will probably never win the Democrats’ love. As hard as it is to stand on principle, it’s a much harder political world for someone who doesn’t believe in anything.